System, device and method for providing call forwarding in dual subscription mode

ABSTRACT

The invention relates to a method and system for providing interworking between a first and a second network of different types, preferably an IMS and a CS (Circuit-Switched) network. A call received by the first network and directed to a terminal unregistered from the first network, is forwarded to the second network, whereas when the call is received by the second network and is directed to a terminal unregistered from the second network, the call is not forwarded to the first network. The same identifier, preferably the same E.164 number, is used in both networks for identifying the called terminal. Preferably, the first network is informed on the registration of the terminal to the second network, the first network using this information as basis for the decision whether or not to route a call to the second network.

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to a system, device and method for providing callforwarding in dual subscription to different types of networks, e.g. apacket-based and circuit-switched network.

More specifically, the invention relates, according to some of theembodiments, to 3GPP (Third Generation Partnership Project) IP (InternetProtocol) Multimedia network, and to call control, e.g. using a HSS(Home Subscriber Server) or Call Processing Server(CPS).

When considering to enable roaming of a user equipment between networksof different types, e.g. between an IMS (IMS; IP Multimedia Subsystem)and a CS (Circuit-Switched) network, by providing two subscriptions,problems may arise. First, if both subscriptions are unregistered and CSor IMS network receives a call, a forwarding loop may be built: e.g.CS→IMS→CS→IMS→etc until the network detects and interrupts the loop.Second, if a service such as “Call forwarding on mobile not reachable”contains on both sides (IMS and CS) a routing number to another side, itis not possible to route the call e.g. to voice mail at all in “notreachable” case.

SUMMARY OF THE INVENTION

The present invention provides a system, method and/or device as definedin any of the independent claims or any of the dependent claims.

The invention presents simple and effective solutions for the abovediscussed dual subscription problem and provides interworking(“roaming”) between different types of networks, e.g. IMS (IMS; IPMultimedia Subsystem) and CS (Circuit-Switched) networks, e.g. in a casewhere the same identifier such as E.164 number is used on both sides.

No changes are needed in CS network, e.g. GSM (Global System for MobileCommunications) and especially in HLR (Home Location Register). Thesolutions include forwarding only from one network, e.g. IMS, to theother network, e.g. GSM, but not vice versa. The solutions use a synonymof the original number as routing number. In some of the embodiments,the functionality of a mobile terminal is additionally used.

According to one aspect, the invention provides a simple solution to astand-alone case (HLR and UMS, User Mobility Server, are independent andhave no interface between them).

According to one of the preferred implementations of the invention,there is provided a method and/or system for providing interworkingbetween a first and a second network of different types, wherein aterminal can be registered to the first network and/or to the secondnetwork, wherein a call received by the first network and directed to aterminal unregistered from the first network, is forwarded to the secondnetwork, whereas when the call is received by the. second network and isdirected to a terminal unregistered from the second network, the call isnot forwarded to the first network.

According to another one of the preferred implementations of theinvention, there is provided a method and/or system for providinginterworking between a first and a second network of different types,wherein a subscriber entity can be registered to the first networkand/or to the second network, wherein a call received by the firstnetwork and directed to a subscriber entity unregistered from the firstnetwork, can be forwarded to the second network,

-   -   the said forwarding step comprises the following steps:    -   issuing an enquiry to service execution means, the said enquiry        carrying call characteristics,    -   the said service execution means arranged to be aware of the        conversion capabilities of a gateway means providing media        stream conversion between said first and second network,    -   the said service execution means inspecting call        characteristics,    -   the said service execution means making a decision as to whether        the call characteristics are such that at least one media stream        related to the call can be converted by the said gateway means,    -   issuing from the said service execution means a forwarding        request to the second network based on the said decision.

The subscriber entity may be a SIM or USIM card or other subscriptioninformation associated or attachable to a mobile equipment at a givenmoment in time.

According to another one of the preferred implementations of theinvention, there is provided a method and/or system for providinginterworking between a first and a second network of different types,wherein a terminal can be registered to the first network and/or to thesecond network, wherein a session request received by the first networkand directed to a terminal unregistered from the first network, isforwarded to the second network together with an information indicatingthat the session request was forwarded from the first network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a basic structure of a communication system inaccordance with an embodiment of the invention,

FIG. 2 shows an alternative message flow in the embodiment of theinvention,

FIG. 3 illustrates a basic structure of a communication system inaccordance with another embodiment of the invention, and

FIG. 4 shows an alternative message flow in the embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Basically, the invention provides a configurable system and method fordual subscription to different types of networks, e.g. a packet-based,e.g. IP-based, and a circuit-switched network. The embodiments relate todual subscription to two networks of different types, i.e. networkinterworking such as IMS-CS interworking.

Both embodiments according to FIGS. 1 and 2 provide call or messageforwarding only to one direction, that is from one network to anothernetwork of another type, but not vice versa. Some embodimentsadditionally implement the forwarding only to one direction with thehelp of a dual-mode terminal that can register into at least twodifferent types of network e.g. IMS and GSM.

FIG. 1 illustrates a first embodiment of the invention which includes auser equipment 11, e.g. a mobile terminal such as a cellular phone orportable computer which is registrable and attachable to a first network12, preferably a packet-based network such as an IMS (IP MultimediaSubsystem) network. The IMS network 12 is the home IP network of thesubscriber B of terminal 11.

The network 12 comprises an I-CSCF (Interrogating Call State ControlFunction) 13, a HSS (Home Subscriber Server) 14, an S-CSCF (Serving CallState Control Function) 15, a service entity sometimes also termedservice cloud 16 which can be a system, method, device, and/or server oralike that can offer services, a BGCF (Breakout Gateway ControlFunction) 17, and a MGCF (Media Gateway Control Function) 18.

The entity 16 can be an Application Execution Environment (AExE) whichreceives, from S-CSCF 15, information on the session or callcharacteristics such as Quality of Service (QoS), admissible delay,jitter, etc. More generally, the session characteristics are a so-calledsession description describing the session parameters such as the onesmentioned above, and/or defining the session to be a video call whichmay contain video stream and audio stream, an audio call, a stereo audiocall etc. The session characteristics may also contain a sessiondescription defining the type of coding such as audio coding AMR.

The terminal 11 may additionally or alternatively be registrable orattachable to a GSM network 20 which includes a HLR (Home LocationRegister) 21 and a GMSC (Gateway Mobile services Switching Center) 22.In the shown case, the GSM network 20 is the home GSM network of thesubscriber B of terminal 11. Further, a PSTN (Public Switched TelephoneNetwork) 23 is shown in FIG. 1.

The embodiments according to the invention present severalimplementations described below which may be provided alternatively orcumulatively.

In the case of FIG. 1, the subscriber B, i.e. the terminal 11, isunregistered, or out of coverage, from the home IMS network 12, and acall to terminal 11 received by IMS network 12 is forwarded to GSMnetwork 20.

According to a specific embodiment, when the IMS subscription ofterminal 11 is unregistered or the terminal 11 is out of coverage, andthe IMS network 12 receives a call to terminal 11, the call is routed toGSM network 20 only when the IMS network 12 deduces of informationavailable that the user might be registered in GSM network 20.

According to another implementation of the invention, the mobileterminal 11 knows after every location update that it is registered inGSM network 20. This information can e.g. be transported, for instanceafter every location update, to the IMS network 12, where it is used asthe basis for the decision whether or not to route the call to GSMnetwork 20 if the terminal 11 loses coverage or becomes unregisteredfrom IMS network 12.

According to a further implementation, a new service is provided in IMSnetwork 12 that could be called for instance: “Conditional sessionforwarding on mobile not reachable”.

Further, for routing a call from IMS network 12 to GSM network 20, arouting number, which is a synonym of the original identifier of thesubscriber B, is used to route the call from IMS to GSM. The originalidentifier is e.g. an E.164 number or a logical name likejohn.smith@ims.sonera.fi. The synonym may or may not be the same as theoriginal E.164 identifier or part of the original E.164.

Note that an E.164 number is a number identifying a called party orterminal and being structured in accordance with the standard ITU-T Rec.E.164 (international public telecommunication numbering plan). An E.164number typically consists of three fields, CC (country code), NDC(national destination code), and SN (subscriber number).

In each of the above implementations, when the GSM subscription ofterminal 11 is unregistered or the terminal 11 is out of coverage in GSMnetwork, and the GSM network 20 receives a call to terminal 11, the callis not routed to IMS network 12.

The embodiment shown in FIG. 1 illustrates a case in which the terminal11, i.e. subscriber B, is unregistered from IMS 12, and a call ispossibly forwarded from IMS network 12 to GSM network 20. In thisembodiment, an E.164 (that is an IMS and GSM number) number is used asidentifier for calling a terminal.

In step 1., a call setup message, e.g. an INVITE message of SIP (SessionInitiation Protocol), is sent from a terminal 11′ to I-CSCF 13 forsetting up a call to terminal 11. The call setup message indicates anidentifier for identifying terminal 11, e.g 358401223344@ims.sonera.fiindicating the E.164 number of terminal 11.

Step 2: The I-CSCF 13 sends a Location request to HSS 14 including theidentifier of terminal 11, e.g. 358401223344@ims.sonera.fi.

Step 3: Because the subscriber is not registered in IMS, the HSS 14indicates this fact to the I-CSCF 13, and may or may not assign a S-CSCFto the call.

Step 4: The I-CSCF 13 chooses a S-CSCF 15 for the call if the HSS didnot assign it and sends a message to S-CSCF 15, e.g. an INVITE messagewith the identifier 358401223344@ims.sonera.fi.

Step 5: S-CSCF 15 makes a query to the service cloud 16 with theoriginal number or with a number fetched from the subscriber's profileand asks for the service “Session forwarding on mobile not reachable”.

Step 6: The service cloud 16 returns a number that is a synonym of theoriginal identifier, i.e. number, e.g. 358491223344. This synonym numberis appropriate as a GSM number and is used as routing number.

Step 7: S-CSCF 15 sends a message, e.g. an INVITE message, to BGCF 17and indicates the routing number 358491223344 in the INVITE message.

Step 8: The BGCF 17 chooses the correct MGCF 18 and sends the INVITEmessage to MGCF 18 indicating the routing number. MGCF 18 translates thesynonym i.e. the routing number to the original E.164 number i.e.358401223344 if the original E.164 identifier is not conveyed to MGCF.

Step 9: The MGCF 18 sends a setup request to GMSC 22 indicating theE.164 number 358401223344.

Step 10: GMSC 22 sends a Location request to HLR 21 with 358401223344.The HLR 21 returns, if available, the requested information.

When the terminal 11 can be reached, the call is setup to terminal 11,otherwise, the call is handled normally as a call to an unregisteredsubscriber.

In the following, an alternative functioning of the embodiment of FIG. 1is described wherein another identifier for identifying a calledsubscriber/terminal is used. In this example, a Universal ResourceLocator (URL) such as SIP-URL is used. The step numbering shown in FIG.1 applies to this embodiment as well.

Step 1: A call setup request message such as an INVITE message of SIP issent from a call originating terminal 11′ to network 12, the messageincluding the identifier e.g. john.smith@ims.sonera.fi of the calledterminal.

Step 2: I-CSCF 13 sends a Location request to HSS 14 which requestincludes the identifier john.smith@ims.sonera.fi.

Step 3: Because the subscriber is not registered in IMS network 12, theI-CSCF 13 is informed thereon, and S-CSCF may or may not be assigned tothe subscriber. I-CSCF 13 then chooses itself a S-CSCF 15 for the callif the HSS did not assign it.

Step 4: I-CSCF 14 sends an INVITE message to the selected S-CSCF 15 withjohn.smith@ims.sonera.fi.

Step 5: S-CSCF 15 makes a query to the service cloud 16 indicating theidentifier john.smith@ims.sonera.fi or a number fetched from thesubscriber's profile and asking for the service “Session forwarding onmobile not reachable”.

Step 6: The service cloud 16 returns a number that is a synonym of theoriginal identifier and preferably is the E.164 number, e.g.358491223344. This synonym number is appropriate as a GSM number and isused as routing number.

Steps 7 to 10 are similar to the above described steps 7 to 10.

In these or other embodiments, a new service like “Voice call forwardingon mobile not reachable” or “Conditional session forwarding on mobilenot reachable” is provided. In this embodiment, one of the components ofthe IMS network 12, e.g. S-CSCF 15, or preferably the AExE 16, checks atleast QoS (Quality of Service) requirements e.g. whether the connectionrequest to terminal 11 is for instance a voice call request or anotherrequest type, e.g. packet-based connection or data connection. A sessionforwarding service is continued only when detecting, in this checkingstep, that the requested connection has such QoS requirements (e.g.voice call) that it can be routed to CS network. This ensures that onlythose sessions that can successfully be forwarded to GSM (at least QoSis preferably checked), are actually forwarded to GSM.

FIG. 2 shows a case where the subscriber B, i.e. the called terminal 11is unregistered from the home GSM network 20, and a call to terminal 11is not forwarded from GSM network 20 to the packet-based network, e.g.home IMS network 12 even when B should be registered in the home IMSnetwork 12.

In this case, an E.164 number (that is an IMS and GSM number) is sentfrom the call originating terminal 11′ for identifying B.

In a step 1 a, a setup request may be issued from PSTN 23 to GMSC 22indicating the identifier, e.g. E.164 number, i.e. +35B-40-1223344.

Alternatively, in a step 1 b: A setup request indicating the E.164number of the called terminal 11, e.g. +358-40-1223344, is sent fromnetwork 12, i.e. from MGCF 18, to GSM network 20 because the user isunregistered in IMS network 12.

Step 2: The GMSC 22 sends a Location request to HLR 21 with+358-40-1223344.

Step 3: Because the subscriber is not registered in GSM, HLR 21 returns,if available, a “Call forwarding on mobile not reachable” number that isa number e.g. to voice mail. The call is then directed to the voicemail. Otherwise, the call is released.

The described embodiments have the advantages of simple structure, anddo not require changes of the GSM network. In particular when the GSMsubscription is the primary subscription for the user, i.e. the usernormally is not registered only in IMS but also in GSM, the greatestpart of the calls will be successfully routed.

According to one of the aspects of the invention, when the S-CSCF 15 inthe user's home core network such as network 12 receives a sessionset-up request including the session or call characteristics, it sendsthis request or at least a request including the session characteristicsto the entity (e.g. AExE) 16 which checks, at least when the terminatingterminal is unregistered from network 12, the session characteristicsfor deciding whether or not this session can be routed, i.e. forwardedto the CS-network 20. As an example the entity 16 checks whether themedia gateway 18 is able to translate the traffic content of therequested session to a form appropriate for forwarding it to the CSnetwork 20. E.g. the audio/speech coding like AMR of the session to beset-up, and the possibility of the media gateway to convert the speechcoding to PCM coding suitable for network 20, is checked by entity 16.If no proper conversion is available, the entity 16 decides not toforward the session, and informs the CSCF 15 to release the session.

Generally speaking, the network 12, e.g. entity 16, inspects, at leastwhen the terminal 11 is unregistered from the network 12, the sessioncharacteristics for checking the possibility of successfully forwardingthe session to the CS network 20 after properly converting the sessiontraffic or signaling in the media gateway 18. When detecting animpossibility of proper conversion, e.g. Stereo Audio to PCM (mono)audio, the session is released without forwarding it.

When the network 12, e.g. entity 16, detects that the session, e.g. avideo call or video stream can not be successfully routed to CS network20, the network 12, e.g. entity 16, can directly route such a session toan appropriate entity such as a Video Mail System when the terminatingterminal 11 is unavailable.

As an alternative, the network 12 can be structured to route a sessiondirected to an unregistered terminal 11 directly from S-CSCF 15 to themedia gateway 18. Only when the gateway 18 returns an error message tothe S-CSCF 15 because of lack of conversion of the traffic and/orsignaling, the S-CSCF 15 addresses entity 16 and requests a decision asto how to further proceed with the requested session. The entity 16 thenanalyzes the session characteristics and instructs the network 12, e.g.CSCF 15, to forward, if possible, the session e.g. to an appropriatemail system such as video mail system.

The interface between S-CSCF 15 and entity 16 can be or use SIP+ or canbe implemented as a Service Control Point (SCP).

In a preferred embodiment which may include the above mentionedfeatures, the terminal 11 is a dual-mode terminal. This embodiment isexplained with regard to FIGS. 1, 2 again.

In this embodiment, when the IMS subscription of the called party isunregistered, and the IMS network receives a call to this party, thecall is routed to GSM network only when the network deduces of theinformation available that the user might be registered in GSM and QoS(Quality of Service) requirement of the call is suitable for successfulrouting to GSM.

However, when the GSM subscription of terminal 11 is unregistered, andthe GSM network receives a call for terminal 11, the call is not routedto the IMS network.

Alternatively, or additionally, the mobile terminal knows after everylocation update that it is registered in GSM. This information istransported for instance after every location update to IMS network,where it is used as the basis for the decision whether or not to routethe call to GSM if the terminal loses coverage or becomes unregisteredfrom the IMS network. A new service is provided in IMS: “Conditionalsession forwarding on mobile not reachable”.

In the present case, when B is unregistered from IMS, a call is possiblyforwarded to GSM. The routing number preferably is a synonym of theoriginal identifier and is used to route the call from IMS to GSM. Inthis implementation, the E.164 number (that is an IMS and GSM number) isused as identifier of the call terminating terminal 11′.

Steps 1 to 4 of this embodiment are identical to the steps defined abovewith regard to the E.164 number embodiment.

Step 5: S-CSCF 15 checks QoS (Quality of Service) requirements of thecall whether the call is a voice call or other type of call that can besuccessfully routed to GSM and makes a query to the service cloud 16with the original number or with a number fetched from the subscriber'sprofile asking “Conditional session forwarding on mobile not reachable”.

Step 6: The service cloud 16 returns, if available, a number that is asynonym of the original identifier, e.g. 358491223344. This synonymnumber is used as routing number.

S-CSCF 15 checks the information available in order to deduce whetherthe subscriber is registered in GSM. If this is not the case, the S-CSCF15 makes a further query to the service cloud with the originalidentifier asking “Session forwarding on mobile not reachable” androutes the session to the received destination, e.g. to voice mail.

If the subscriber is registered in GSM, the process goes to the nextsteps 7 and following as described above.

In this embodiment, information is provided for IMS about terminalregistration in GSM in one or more of the following manners.

A. After each successful location update the dual terminal sends amessage/information to the IMS network 12. The IMS network records thetime of the successful location update in GSM e.g. “registered in GSM at999999”.

B. When the user switches off his dual terminal, the terminal sends amessage/information to the IMS network 12 indicating that it is switchedoff. The IMS network records the state “switched off” i.e. “unregisteredfrom GSM”.

C. When the user unregisters from the GSM network, the dual terminalsends a message/information to IMS network 12 that it will unregisterfrom GSM. The IMS network 12 records the state “unregistered from GSM”.

D. Valid information about registration in GSM is continuously availablein HSS 14.

The user may prevent sending message/information of unregistration fromGSM and/or switching off the terminal if he e.g. is going to roam to adistrict, country or alike where IMS network is not available. This mayhappen e.g. when the user switches off his terminal before boarding anairplane and he knows that in the destination only GSM network isavailable.

This information is preferably used as follows.

When the terminal is not registered in IMS, it is checked whether theterminal is registered in GSM. The information gathered from theterminal is utilized. At least the following alternative rules may befollowed according to the operator's preferences:

A rule 1 can be: If the terminal is “unregistered from GSM”, the call isnot forwarded to GSM, otherwise it is.

A rule 2 may be: If the terminal is “registered in GSM at 999999” andthe time elapsed since that registration time is not longer than acertain period, the call is forwarded to GSM, otherwise it is not.

An additional advantage of this solution, in particular compared to theabove discussed solution, is that the user can decide very simplywhether voice calls are forwarded from IMS to GSM. If he switches offthe terminal or unregisters from GSM, the calls are not forwarded toGSM; if he unregisters from IMS or is outside radio coverage from IMSthe calls are forwarded to GSM.

Some functionality is provided in IMS to keep the information aboutregistration in GSM.

FIGS. 3, 4 show further embodiments where a session, e.g. a call isforwarded to both directions, i.e. from network 12 to network 20, orfrom network 20 to network 12.

The functionality of CS network 20 is not changed i.e. the IMS network12 works together with a GSM network of customary structure. The sameidentifier, e.g. the same E.164 number identifies the subscription inIMS and GSM. A dual terminal may be used.

According to some aspects of these embodiments, when the IMSsubscription is unregistered, and the network 12 receives a sessionrequest, e.g. a call, the session is routed to GSM network 20. When theGSM subscription is unregistered, and the network 20 receives a sessionrequest, e.g. a call, the session is routed to IMS network 12. A routingnumber or identifier, which may be a synonym of the original E.164, isused to route the session from IMS 12 to GSM network 20. A routingnumber or identifier, which may be another synonym of the originalE.164, is used to route the session from GSM 20 to IMS 12 network.

The functionality at I-CSCF 13 in IMS network 12 is as follows.

1) Do the UMS query always with the original identifier, e.g. theoriginal E.164, and translate, if needed, the identifier, e.g. E.164from synonym (i.e. the routing number) to the original identifier, e.g.E.164.

2) If the subscriber is registered in UMS, route the session, e.g. call,to the correct S-CSCF 15.

3) If the subscriber is unregistered in UMS, choose a S-CSCF and routethe session there with information that the subscriber is unregistered.

The functionality at S-CSCF 15 in IMS network 12 is as follows.

1) If the subscriber is registered in UMS, translate the identifier,e.g. E.164 to the original one when needed, consult the service cloud 16and route the session, e.g. call according to the given advice, normallyto the subscriber.

2) If the subscriber is unregistered in UMS:

-   -   if the E.164 is the original E.164 (i.e. the session e.g. call        is not coming from GSM network 20) and the QoS of the session is        such that the session can be routed or is reasonable to route to        GSM, translate the E.164 to a GSM synonym number and forward the        call to GSM 20 with it.

The service cloud need not be consulted. If it is consulted, the advice“Session forwarding on mobile not reachable” is not followed.

-   -   if the identifier, e.g. E.164, is the IMS synonym number (i.e.        the session e.g. call is coming from GSM network 20), translate        the identifier when needed, e.g. E.164 to the original number,        consult the service cloud and follow the advice “Session        forwarding on mobile not reachable” if available, or release the        call if no advice is available.

3) If the call is released and the E.164 is the GSM synonym number i.e.the GSM network 20 releases the call (because e.g. the subscriber isunregistered in GSM), translate the E.164 to the original number whenneeded, consult the service cloud 16 and follow the advice “Sessionforwarding on mobile not reachable” if available, or release the call ifno advice is available.

The functionality at GMSC 22 in CS network 20 is as follows:

1) Do the HLR 21 query always with the original identifier, e.g. E.164.

-   -   Translate the E.164 from synonym (i.e. the routing number) to        the original E.164 if needed.

2) If the subscriber is registered in HLR 21, translate the E.164 to theoriginal number when needed and route the session with the routingnumber received from HLR 21.

3) If the subscriber is unregistered in HLR 21:

-   -   if the identifier, e.g. E.164 is the original identifier, e.g.        E.164 (i.e. the session is not coming from IMS 12), translate        the identifier to the IMS synonym number and forward the call to        IMS 12 with this synonym number. Possible advice “Call        forwarding on mobile not reachable” from IN, Intelligent        Network, (or somewhere else) is not followed.    -   if the identifier, e.g. E.164, is the GSM synonym number (i.e.        the call is coming from IMS 12), translate the identifier to the        original number when needed, possibly consult IN (or possibly        some other information source) and follow the advice “Call        forwarding on mobile not reachable” if available, or release the        session if no advice is available.

4) If the session is released and the identifier, e.g. E.164 is the IMSsynonym number i.e. the IMS network 12 releases the session (becausee.g. the subscriber is unregistered in IMS 12), translate the identifierto the original number when needed, possibly consult IN (or possiblysome other information source) and follow the advice “Call forwarding onmobile not reachable” if available, or release the session if no adviceis available.

Advantages of these solutions according to FIGS. 3, 4 are:

a) Quite simple,

b) No changes to GSM network needed, only routing has to be slightlyreconfigured,

c) Offers the possibility to prioritize one of the networks according tothe subscriber's preferences, e.g. have voice mail only in GSM 12 andnot on both networks.

In the examples according to FIGS. 3, 4:

-   the original identifier is the E.164 number: 358-40-1223344-   the GSM synonym is: 358-49-1223344-   the IMS synonym is: 358-48-1223344-   FQDN of the operator's IMS: ims.sonera.fi.

The embodiment shown in FIGS. 3 and 4 comprises the structuralcomponents as shown and described with regard to FIGS. 1, 2, andadditionally a voice mail component/device or function 24.

First, the functioning of this embodiment will be explained withreference to the arrow numbering of FIG. 3, and the use of an identifierwhich is the E.164 number (that is an IMS and GSM number).

Step 1: INVITE message to I-CSCF 13 from calling terminal 11′ withidentifier 358401223344@ims.sonera.fi.

Step 2: Location request to HSS 14 with 358401223344@ims.sonera.fi.

Step 3: Because the subscriber is not registered in IMS, HSS 14 does notassign a S-CSCF to the session, e.g. call. The I-CSCF 13 chooses aS-CSCF 15 for the session.

Step 4: INVITE message to the selected S-CSCF 15 with358401223344@ims.sonera.fi and with information that the subscriber isunregistered in the IMS network 12.

Step 5: The S-CSCF 15 may make a query to the service cloud 16 with theoriginal identifier 358401223344@ims.sonera.fi.

Step 6: The service cloud 16 may return advice “Session forwarding onmobile not reachable”. The advice is not followed. The GSM synonym358-49-1223344 is generated by S-CSCF 15 from the original identifier,E.164.

Step 7: S-CSCF 15 sends an INVITE message further to BGCF 17 with theGSM synonym +358-49-1223344.

Step 8: BGCF 17 chooses the correct MGCF 18 and sends an INVITE messageto it.

Step 9: MGCF 18 sends a setup request to GMSC 22 with the GSM identifier+358-49-1223344.

Step 10: A Location request indicating +358-40-1223344 (i.e. theoriginal E.164) is generated by GMSC 22 and sent to HLR 21. Because thesubscriber is unregistered in HLR 21, an IN (or some other informationsource) is consulted, and the advice “Call forwarding on mobile notreachable” is followed if available, otherwise the session is released.

Step 11: The session is routed e.g. to voice mail 24 with the originalE.164.

The solution can also incorporate, in IMS network 12, a service like“Voice call forwarding on mobile not reachable”. Then only those callsthat can be forwarded to GSM 20 (at least QoS is checked) are forwardedthere.

When-the identifier used in the embodiment of FIG. 3 is e.g. SIP-URLinstead of E.164 number, the following steps may be performed:

Step 1: INVITE to I-CSCF 13 with john.smith@ims.sonera.fi.

Step 2: Location request from I-CSCF 13 to HSS 14 withjohn.smith@ims.sonera.fi.

Step 3: Because the subscriber is not registered in IMS, no S-CSCF 15 isassigned to the subscriber. HSS 14 returns the original E.164 number toI-CSCF 13. I-CSCF 13 chooses a S-CSCF 15 for the session.

Steps 4-11 are like above except in the step 4 and/or 5 the S-CSCFfetches the original E.164 number from the subscriber's profile if HSSdid not returned it.

The embodiment of FIG. 4 illustrates the method flow in case ofreceiving a session request by the network 20. First, a case isexplained when the used identifier is the E.164 number (that is an IMSand GSM number).

Step 1: A Setup request is sent to GMSC 22 from a calling terminal 11′with identifier +358-40-1223344.

Step 2: A Location request with this identifier +358-40-1223344 (i.e.with the original E.164) is sent from GMSC 22 to HLR 21. The calledsubscriber is unregistered in HLR 21. Because the identifier E.164 isthe original number, it is translated by GMSC 22 or HLR 21 to IMSsynonym i.e. +358-48-1223344.

Step 3: GMSC 22 sends a Setup request to MGCF 18 indicating theidentifier 358-48-1223344.

Step 4: MGCF 18 sends an INVITE message to I-CSCF 13 with identifier358-48-1223344@ims.sonera.fi which is generated by MGCF 18 based on thereceived identifier 358-48-1223344 and the IMS identifier ims.sonera.fi.

Step 5: I-CSCF 13 generates an identifier 358401223344@ims.sonera.fi(i.e. with the original E.164) and sends a Location request to HSS 14with this identifier 358401223344@ims.sonera.fi.

Step 6: Because the subscriber is not registered in IMS, no S-CSCF 15 isassigned from HSS 14 to the session. I-CSCF 13 chooses a S-CSCF 15 forthe session.

Step 7: I-CSCF 13 sends an INVITE message to the selected S-CSCF 15 withthe IMS identifier 358481223344@ims.sonera.fi and with information thatthe subscriber is unregistered.

Step 8: S-CSCF 15 makes a query to the service cloud 16 with theoriginal number 358-40-1223344 generated by S-CSCF 15 based on theidentifier received from I-CSCF 13.

Step 9: The service cloud 16 may return advice “Session forwarding onmobile not reachable”. If the advice is returned, it is followed.Otherwise the call is released. In this example the call is released.

Step 10: A Release command is sent from S-CSCF 15 to I-CSCF 13 forreleasing the session 358481223344@ims.sonera.fi.

Step 11: A Release command including the identifier358481223344@ims.sonera.fi is sent from I-CSCF 13 to MGCF 18.

Step 12: A Release command including the identifier 358-48-1223344 issent from MGCF 18 to GMSC 22. GMSC 22 checks and detects that the callis released and the E.164 is the IMS synonym number. GMSC 22 translatesthe E.164 to the original number, possibly consults IN (or possibly someother information source) and follows the advice “Call forwarding onmobile not reachable” if available, or release the call if no advice isavailable. In this example the advice is to route the call to voicemail.

Step 13: The call is routed with 358-40-1223344 to voice mail 24.

In the above embodiments, the functionality of CS network is not changedi.e. the IMS works together with any customary type of GSM network.Further, the same identifier, e.g. the E.164 number, identifies thesubscription in both networks, e.g. IMS and GSM. A dual-mode terminal(Dual terminal) is used which can register to IMS and GSM.

The invention can be implemented to support also other technologies ornetworks than IMS and GSM. The concept “registered in GSM” meansattached in GSM while the concept “unregistered in GSM” means detachedfrom GSM. The names “Session forwarding on mobile not reachable”, “Callforwarding on mobile not reachable”, “Conditional session forwarding onmobile not reachable”, “Voice call forwarding on mobile not reachable”,and alike should not be considered as exact names but descriptive namesof service, functionality, functional procedure or alike.

Although the invention has been described above with reference tospecific embodiments, the scope of the invention also covers anyalterations, additions, modifications, and omissions of the disclosedfeatures.

1. A method, comprising: providing interworking between a first wirelesstelecommunication network and a second wireless telecommunicationnetwork, the first wireless telecommunication network and the secondwireless telecommunication network being of different network types,wherein one of the first wireless telecommunication network and thesecond telecommunication network is an Internet protocol based networkand the other wireless telecommunication network is a circuit-switchednetwork; registering a dual-mode mobile terminal to one of the firstwireless telecommunication network and the second wirelesstelecommunication network; forwarding a session request received by thefirst wireless telecommunication network and directed to a terminalunregistered from the first wireless telecommunication network to thesecond wireless telecommunication network; and checking whether thesession request directed to the terminal is a request that can besuccessfully routed to the second wireless telecommunication network,wherein the forwarding is continued only when the checking determinesthat the session request can be successfully routed to the secondwireless telecommunication network.
 2. The method according to claim 1,wherein the first wireless telecommunication network is an Internetprotocol based network, and the second wireless telecommunicationnetwork is a circuit-switched network.
 3. The method according to claim2, wherein the first wireless telecommunication network comprises anInternet protocol multimedia subsystem network.
 4. The method accordingto claim 1, wherein a same identifier is used in both the first wirelesstelecommunication network and the second wireless telecommunicationnetwork, the same identifier being configured to identify a calledterminal.
 5. The method according to claim 1, wherein the dual-modemobile terminal is informed after every location update on itsregistration in the second wireless telecommunication network.
 6. Themethod according to claim 1, further comprising: informing the firstwireless telecommunication network of a registration of the dual-modemobile terminal to the second wireless telecommunication network,wherein the first wireless telecommunication network is configured touse the information as a basis for a decision whether or not to routethe session request to the second wireless telecommunication network. 7.The method according to claim 1, further comprising: sending amessage/information to the first wireless telecommunication network fromthe second wireless telecommunication network after a location update inthe second wireless telecommunication network, wherein the firstwireless telecommunication network is configured to record a time of thelocation update.
 8. The method according to claim 1, further comprising:sending a message/information to the first wireless telecommunicationnetwork when the dual-mode mobile terminal is switched off, themessage/information indicating a switching off and unregistering of thefirst wireless telecommunication network from the second wirelesstelecommunication network, wherein the first wireless telecommunicationnetwork is configured to record an unregistered state.
 9. The methodaccording to claim 1, further comprising: sending a message/informationto the first wireless telecommunication network that the dual-modemobile terminal will unregister from the second wirelesstelecommunication network, wherein the first wireless telecommunicationnetwork is configured to record an unregistered state.
 10. The methodaccording to claim 1, wherein information about registering thedual-mode mobile terminal in the first wireless telecommunicationnetwork is continuously available in a server.
 11. The method accordingto claim 1, further comprising: checking whether the dual-mode mobileterminal is registered in the second wireless telecommunication networkwhen the dual-mode mobile terminal is not registered in the firstwireless telecommunication network.
 12. The method according to claim11, further comprising: utilizing information gathered from thedual-mode mobile terminal for checking whether the dual-mode mobileterminal is registered in the second wireless telecommunication network.13. The method according to claim 1, wherein the forwarding comprisesdetecting whether the dual-mode mobile terminal is unregistered from thesecond wireless telecommunication network, and the method furthercomprises forwarding the session request to the second wirelesstelecommunication network when detecting that the dual-mode mobileterminal is registered to the second wireless telecommunication network.14. The method according to claim 1, further comprising: checking a timeduration elapsed since a time point of registration of the dual-modemobile terminal to the second wireless telecommunication network; andforwarding the session request to the second wireless telecommunicationnetwork when the time duration is shorter than a certain period,otherwise the session request is not forwarded.
 15. The method accordingto claim 1, wherein the first wireless telecommunication network isconfigured to provide a session forwarding service used when asubscriber is not reachable in the first wireless telecommunicationnetwork and the session request characteristics indicate that thesession request can be successfully routed to the second wirelesstelecommunication network.
 16. The method according to claim 15, whereinthe session forwarding service comprises a service for conditional voicecall forwarding on a mobile not reachable in the first wirelesstelecommunication network.
 17. The method according to claim 15, whereinone of the components of the first wireless telecommunication network isconfigured to check whether a connection request to the dual-mode mobileterminal is a voice call request or a request for a packet-basedconnection or a data connection, and wherein the session forwardingservice is continued only when detecting that the connection requestrepresents the voice call request.
 18. The method according to claim 1,wherein the first wireless telecommunication network is configured toinspect, at least when the dual-mode mobile terminal is unregisteredfrom the first wireless telecommunication network, sessioncharacteristics of an intended session for checking a possibility ofsuccessfully forwarding the session to the second wirelesstelecommunication network.
 19. The method according to claim 1, furthercomprising, when the first wireless telecommunication network receives asession set-up request comprising session or call characteristics,sending the session set-up request or at least a request comprising thesession characteristics to an entity which checks, at least when aterminating terminal is unregistered from the first wirelesstelecommunication network, the session characteristics for decidingwhether or not the one of the session set-up request and the requestcomprising the session characteristics can be routed to the secondwireless telecommunication network.
 20. The method according to claim 1,wherein an entity of the first wireless telecommunication network checkswhether a media gateway of the first wireless telecommunication networkis configured to translate traffic content of the requested session to aform appropriate for forwarding the traffic content to the secondwireless telecommunication network, and, when no proper conversion isavailable, the entity decides not to forward the session.
 21. The methodaccording to claim 1, wherein, when the first wireless telecommunicationnetwork detects that the session cannot be successfully routed to thesecond wireless telecommunication network, the first wirelesstelecommunication network routes session to a video mail system.
 22. Themethod according to claim 1, wherein the first wirelesstelecommunication network is configured to route a session directed toan unregistered terminal directly to a media gateway, and, when themedia gateway returns an error message because of lack of conversion ofone of traffic and signaling of the session, an entity of the firstwireless telecommunication network is addressed which decides how tofurther proceed with the requested session.
 23. The method according toclaim 22, wherein the entity analyzes session characteristics andinstructs the first wireless telecommunication network to forward, ifpossible, the session to a video mail system.
 24. The method accordingto claim 1, wherein an interface between a serving call session controlfunction and an entity of the first wireless telecommunication networkuses a session initiation protocol or is implemented as a servicecontrol point.
 25. A system, comprising: a first wirelesstelecommunication network; a second wireless telecommunication network,the second wireless telecommunication network being a different networktype from the first wireless telecommunication network; and a dual-modemobile terminal configured to be registrable to at least one of thefirst wireless telecommunication network and the second wirelesstelecommunication network, wherein one of the first wirelesstelecommunication network and the second wireless telecommunicationnetwork is an Internet protocol based network and the other wirelesstelecommunication network is a circuit-switched network, wherein thesystem is configured to forward a session request received by the firstwireless telecommunication network and directed to a terminalunregistered from the first wireless telecommunication network, to thesecond wireless telecommunication network, wherein the first wirelesstelecommunication network is configured to check whether the sessionrequest directed to the terminal is a request that can be successfullyrouted to the second wireless telecommunication network, and wherein thesystem is further configured to continue the forwarding only whendetecting that the session request represents a request that can besuccessfully routed to the second wireless telecommunication network.26. The system according to claim 25, wherein the first wirelesstelecommunication network is an Internet protocol-based network, and thesecond wireless telecommunication network is a circuit-switched network.27. The system of claim 26, wherein the first wireless telecommunicationnetwork comprises an Internet protocol multimedia subsystem network. 28.The system according to claim 25, wherein a same identifier is used inboth the first wireless telecommunication network and the secondwireless telecommunication network, the same identifier being configuredto identify a called terminal.
 29. The system according to claim 25,wherein the system is further configured to inform the dual-mode mobileterminal after every location update on its registration in the secondwireless telecommunication network.
 30. The system according to claim25, wherein the system is further configured to inform the firstwireless telecommunication network on the registration of the dual-modemobile terminal to the second wireless telecommunication network,wherein the first wireless telecommunication network is configured touse the information as a basis for a decision whether or not to route asession request to the second wireless telecommunication network. 31.The system according to claim 25, wherein after each successful locationupdate in the second wireless telecommunication network, the dual-modemobile terminal is further configured to send a message/information tothe first wireless telecommunication network, wherein the first wirelesstelecommunication network is configured to record a time of thesuccessful location update.
 32. The system according to claim 25,wherein the dual-mode mobile terminal is further configured to send,when the dual-mode mobile terminal is switched off, amessage/information to the first wireless telecommunication networkindicating a switching-off, and wherein the first wirelesstelecommunication network is configured to record an unregistered state.33. The system according to claim 25, wherein the dual-mode mobileterminal is further configured to send, when the dual-mode mobileterminal unregisters from the second wireless telecommunication network,a message/information to the first wireless telecommunication networkindicating that the dual-mode mobile terminal will unregister from thesecond wireless telecommunication network, and wherein the firstwireless telecommunication network is configured to record anunregistered state.
 34. The system according to claim 25, wherein thefirst wireless telecommunication network comprises a server configuredto store information about registration of the dual-mode mobile terminalin the first wireless telecommunication network.
 35. The systemaccording to claim 25, wherein the first wireless telecommunicationnetwork is configured to check, when the dual-mode mobile terminal isnot registered in the first wireless telecommunication network, whetherthe dual-mode mobile terminal is registered in the second wirelesstelecommunication network.
 36. The system according to claim 35, whereinthe system is further configured to check whether the dual-mode mobileterminal is registered in the second wireless telecommunication networkusing information gathered from the dual-mode mobile terminal.
 37. Thesystem according to claim 25, wherein the system is further configured,when detecting that the dual-mode mobile terminal is unregistered fromthe second wireless telecommunication network, not to forward sessionrequest to the second wireless telecommunication network, and furtherconfigured, when detecting that the dual-mode mobile terminal isregistered to the second wireless telecommunication network, to forwardthe session request is forwarded to the second wirelesstelecommunication network.
 38. The system according to claim 25, whereinthe system is further configured to check a time duration elapsed sincea time point of registration of the dual-mode mobile terminal to thesecond wireless telecommunication network, and to forward, when the timeduration is shorter than a certain period, the session request to thesecond wireless telecommunication network, which otherwise is notforwarded.
 39. The system according to claim 25, wherein the firstwireless telecommunication network is configured to provide asupplementary service for a conditional voice call forwarding on amobile not reachable in the first wireless telecommunication network.40. The system according to claim 25, wherein the first wirelesstelecommunication network is configured to check whether a connectionrequest to the dual-mode mobile terminal is a voice call request or therequest configured to be successfully routed to the second wirelesstelecommunication network, and to continue call forwarding to the secondwireless telecommunication network only when detecting that theconnection request represents the voice call request.
 41. The systemaccording to claim 25, wherein the first wireless telecommunicationnetwork is configured to provide a session forwarding service used whena subscriber is not reachable in the first wireless telecommunicationnetwork and session request characteristics indicate that the sessionrequest can be successfully routed to the second wirelesstelecommunication network.
 42. The system according to claim 25, furthercomprising: a session forwarding service comprising a service forconditional voice call forwarding on a mobile not reachable in the firstwireless telecommunication network.
 43. The system according to claim25, wherein the first wireless telecommunication network is configuredto inspect, at least when the dual-mode mobile terminal is unregisteredfrom the first wireless telecommunication network, sessioncharacteristics of an intended session for checking a possibility ofsuccessfully forwarding the session to the second wirelesstelecommunication network.
 44. The system according to claim 25,wherein, when the first wireless telecommunication network receives asession set-up request comprising session or call characteristics, thefirst wireless telecommunication network is configured to send one ofthe session set-up request and a request comprising the sessioncharacteristics to an entity which checks, at least when a terminatingterminal is unregistered from the first wireless telecommunicationnetwork, the session characteristics for deciding whether or not the oneof the session set-up request and the request comprising the sessioncharacteristics can be routed to the second wireless telecommunicationnetwork.
 45. The system according to claim 25, wherein an entity of thefirst wireless telecommunication network is configured to check whethera media gateway of the first wireless telecommunication network isconfigured to translate traffic content of the requested session to aform appropriate for forwarding the traffic content to the secondwireless telecommunication network, and, when no proper conversion isavailable, the entity decides not to forward the session.
 46. The systemaccording to claim 25, wherein, when the first wirelesstelecommunication network detects that the session cannot besuccessfully routed to the second wireless telecommunication network,the first wireless telecommunication network is configured to route thesession to a video mail system.
 47. The system according to claim 25,wherein the first wireless telecommunication network is configured toroute a session directed to an unregistered terminal directly to a mediagateway, and, when the media gateway returns an error message because oflack of conversion of one of traffic and signaling of the session, anentity of the first wireless telecommunication network is addressedwhich decides how to further proceed with the requested session.
 48. Thesystem according to claim 47, wherein the entity is configured toanalyze session characteristics and to instruct the first wirelesstelecommunication network to forward, if possible, the session to avideo mail system.
 49. The system according to claim 25, wherein aninterface between a serving call session control function and an entityof the first wireless telecommunication network are configured to use asession initiation protocol or configured to implement a service controlpoint.
 50. A method, comprising: providing interworking between a firstwireless telecommunication network and a second wirelesstelecommunication network, the second wireless telecommunication networkbeing a different network from the first wireless telecommunication;registering a terminal to one of the first wireless telecommunicationnetwork and the second wireless telecommunication network; andforwarding a requested session, received by the first wirelesstelecommunication network and directed to a terminal unregistered fromthe first wireless telecommunication network, to the second wirelesstelecommunication network together with information indicating thesession request was forwarded from the first wireless telecommunicationnetwork for providing interworking between the first wirelesstelecommunication network and the second wireless telecommunicationnetwork.
 51. A method according to claim 50, wherein at least one of thefirst wireless telecommunication network and the second wirelesstelecommunication network are configured to check information and tosuppress the forwarding of the session request.
 52. The method accordingto claim 51, wherein the information comprises an identifier foridentifying the unregistered terminal, the identifier being changed whenforwarding the session request from one of the first wirelesstelecommunication network and the second wireless telecommunicationnetwork to the other of the first wireless telecommunication network andthe second wireless telecommunication network.
 53. A system, configuredto: forward a session request received by a first wirelesstelecommunication network and directed to a terminal unregistered fromthe first wireless telecommunication network, to a second wirelesstelecommunication network; and forward, together with the sessionrequest, information indicating that the session request was forwardedfrom the first wireless telecommunication network for providinginterworking between the first wireless telecommunication network to thesecond wireless telecommunication network, wherein the second wirelesstelecommunication network is a different network type from the firstwireless telecommunication network.
 54. The system according to claim53, wherein the first wireless telecommunication network and the secondwireless telecommunication network are configured to check informationand to suppress the forwarding of a session request.
 55. The systemaccording to claim 53, wherein the information comprises an identifierfor identifying the unregistered terminal, the identifier being changedwhen forwarding the session request from one of the first wirelesstelecommunication network and the second wireless telecommunicationnetwork to the other of the first wireless telecommunication network andthe second wireless telecommunication network.